In long distance optical communication systems it may be important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the optical transmission cable, faulty repeaters or amplifiers, and/or other problems with the system.
In general, known monitoring techniques include use of line monitoring equipment (LME) that generates one or more LME test signals, e.g. at different wavelengths. The LME may transmit the test signals with the information signals, e.g. in a wavelength division multiplexed (WDM) system. The test signals may be returned to the line monitoring equipment through a high-loss loop back (HLLB) path within an amplifier or repeater. The LME may then separate the returned test signals from the data signals, and process the returned test signals to obtain data representing a characteristic of the returned test signals as a metric for characterizing the optical path.
One such monitoring technique involves use of optical time domain reflectometry (OTDR) equipment and techniques. According to conventional OTDR techniques, the LME generates an OTDR test signal that may, for example, be an optical pulse or a specially modulated optical carrier, and launches the OTDR test signal into the outbound optical path of a path pair. Elements in the outbound path may reflect (e.g., backscatter) portions of the OTDR test signal. The backscattered signal portions may be returned (e.g., on the same outbound path or a different path such as the inbound path by coupling through an HLLB) and detected in the LME. The transmission characteristics of each element in the path may also affect the amount of signal reflected at points after that element, for example, by attenuating the test signal or the reflected signal. The magnitude of the backscattered or reflected signal from each element or point along the optical path may be used as a metric for characterizing the optical path.
OTDR techniques include coherent optical time domain reflectometry (COTDR). COTDR uses a special optical modulation scheme for its test signal and a coherent optical detection receiver to improve receiver sensitivity. The improved sensitivity enables measurement of very low levels of backscattered signal and thus the examination of very long optical fibers even if the fibers are in portions of the optical path far from the COTDR equipment (e.g., beyond an optical amplifier). Because Rayleigh backscatter from optical fiber in the transmission path can be detected by OTDR or COTDR, this approach to system monitoring provides a diagnostic tool that allows the user to examine the fiber between repeaters.
Another known line monitoring technique includes inspection of the loop gain of test signals through the HLLB paths within a system. In this approach, the LME may transmit one or more LME test signals representing, for example, a pseudo random bit sequence. The test signals may be returned to the line monitoring equipment through a high-loss loop back (HLLB) path within each amplifier or repeater. The LME may then process the returned test signals to obtain data representing the HLLB loop gain imparted to the test signals in their propagation from the LME, through the HLLB and any intervening optical paths and amplifiers, and back to the LME. Significant deviations in HLLB loop gain may indicate a fault in the system.
OTDR and loop gain monitoring techniques are challenged by the demands of modern long haul communication systems. For example, the cost of an undersea optical cable system and other such long haul communication systems is significantly influenced by the number of repeaters in the system. Thus, there is a continuing desire to expand the spacing between repeaters, so as to reduce the number of repeaters. Although the maximum possible repeater span has increased with improvements such as the introduction of advanced modulation formats, capability of OTDR equipment did not improve in step. In some systems, the reach of OTDR equipment may be limited to within 90 km, so that only about half of a repeater span may be measurable. Moreover, high loss loop back (HLLB) paths in some systems only allow measuring reflected Rayleigh signals from the outgoing direction, because they only have one path connecting from one amplifier output of a repeater to the other amplifier output of that repeater. As such, some conventional architectures may not be able to measure the Rayleigh signal from the incoming fiber path.
With regard to loop gain measurements, it is known that repeater pump power loss and increased fiber span loss may be primary failure mechanisms resulting in HLLB loop gain deviations from normal values. In a known system, significant variations in HLLB loop gain, e.g. above a predefined alarm threshold, may trigger a system alarm. Choice of the alarm threshold in such a system may require discrimination between normal system fluctuations and measurement errors and real transmission path faults. Unfortunately, this discrimination may be difficult since some HLLB loop gain measurements may be generally insensitive to physical changes in the transmission path due, in part, to the repeater loop back output-to-output architecture, as well as gain mechanisms in the repeater amplifier, e.g. self-gain regulation. Consequently, real path changes for non-devastating failures in such systems may result in HLLB loop gain changes in some systems that are only slightly detectable given typical measurement errors and system fluctuations.
One configuration for addressing the limited reach of OTDR is described in U.S. Pat. No. 8,009,983 entitled “High Loss Loop Back for Long Repeater Spans” (the '983 patent), the teachings of which are hereby incorporated herein by reference. The '983 patent describes HLLB configurations that allow bi-directional transmission of OTDR test signals to double the maximum measurable span length using OTDR. One configuration for addressing the sensitivity of loop gain monitoring techniques is described in U.S. Pat. No. 8,135,274 entitled “System and Method for Fault Identification in Optical Communication Systems” (the '274 patent), the teachings of which are hereby incorporated herein by reference. The '274 patent describes a HLLB configuration that increases the sensitivity of the loop gain measurements and compares loop gain measurements against a pre-determined gain signature to identify system faults. Unfortunately, however, the HLLB configuration described in the '983 patent for addressing the limited reach of OTDR does not provide the loop gain sensitivity improvements described in the '274 patent, and the HLLB configuration described in the '274 patent for achieving increased loop gain sensitivity does not support the OTDR reach improvements described in the '983 patent.